1. Field of Invention
The present invention generally relates to a clock generator, and more particularly to a clock generator for providing a spread spectrum output clock signal.
2. Description of Prior Art
In the field of the high speed transmission, the phenomenon in the system, such as error occurrence or abnormally functioning, may happen due to the period signal having the centralized and strong energy. The energy is a form of electromagnetism, and also called as electromagnetic interference (EMI). Recently, a technology of spectrum expansion is used to reduce the electromagnetic interference, which is also called as a spread spectrum technology. The spread spectrum technology makes the frequency of the clock signal distributed in a certain range, so as to diverse the energy of the clock signal, and to reduce the energy of the interference. Generally, the clock generators using the spread spectrum technology can be divided into three categories.
Referring to FIGS. 1A-1C and FIGS. 1A-1C are block diagrams of conventional clock generators of three categories. The spread spectrum clock generator in FIG. 1A receives the input clock signal FIN, and uses the low pass filter (LPF) 110 to perform a direct modulation. Then the spread spectrum clock generator changes the frequency of the output clock signal FOUT by changing the control voltage generated by the low pass filter 110. Most of the spread spectrum clock generators of this category need passive components of the larger size, so as to obtain the better stability control. However the passive components of the larger size consume the larger area and cost, and have the higher sensitivity for the process, the temperature and the voltage.
In FIG. 1B, the delta-sigma phase switching or the phase compensation is used to construct the spread spectrum clock generator 120, and thus the spread spectrum clock generator 120 can adjust the frequency of the output clock signal. The spread spectrum clock generator of this category uses the little phase deviation to perform a modulation, and therefore it has the smaller jitter. However, while operating in the high frequency condition, it is hard to obtain the accurate phase due to the effect of the parasitical capacitor and the parasitical resistor on the unmatched winding wire.
In addition, the spread spectrum clock generator in FIG. 1C is the spread spectrum clock generator of the third category which uses the fractional frequency dividing to perform a modulation. The multimodulus frequency divider 130 thereof has two or more than two moduluses and multiples the frequency of the output clock signal FOUT with a fraction number via the switching control of a certain ratio made by the spread spectrum modulator 140.
Take the spread spectrum clock generator in FIG. 1C as an example, the switch between the several moduluses is needed when the fractional frequency dividing is processed. The switch between the several moduluses is occurred in the fixed period. Though the effect of the average fractional frequency dividing is achieved, the phase detector 150 generates an error signal at the moment of the switch between the several moduluses. The error signal will affect the voltage controlled oscillator 160, change the frequency of the output clock signal FOUT, and generate a fractional spike in the spectrum.
Moreover, the spread spectrum modulator 140 used to control the multimodulus frequency divider 130 is usually constructed by the delta-sigma modulator, and the effect of the spread spectrum is determined by the bit width of the delta-sigma modulator. The higher the bit width is, and the smaller the quantization error can be obtained. Referring to FIG. 2, FIG. 2 is a wave diagram of the spread spectrum control voltage signal. The larger the bit width of the sigma-delta modulator is, the higher resolution thereof can be obtained, and the smoother the triangle wave 210 is. Therefore, a better effect of the spread spectrum is achieved.